To tell the truth, there's no exact treatment for visual snow. In some cases, visual snow is due to migraine's complication. In addition, optic neuritis caused by multiple sclerosis can also lead to visual snow. Also, it is said that some self treatments to certain diseases, including lyme disease, auto-immune disease, noxious event may also cause visual snow. However it is not certain. In fact, there is no certain cause to visual snow. Hallucinogen persisting perception disorder (HPPD) is another cause to visual snow. If your father's visual snow is caused by HPPD, clonzepam is the first choice. So, I suggest your father should go to see the doctor to see the exact cause to visual snow.
People with visual snow may see snow or television-like static in part or the whole of their visual field. It is like camera noise in low light conditions. And as far as i know, there is still no treatment for them. But some patient are recommended to use clonazepam in HPPD. I am not sure if it work since i didn't get visual snow and have no experience to use it. Anyway, i suggest you get a completely eye examination and consult your eye doctor. S/he may give you some useful idea to treat it.
Your father's visual snow for days must have been caused by the bad habits of using your eyes. There is a cure for visual snow. You could use the eye drops with anti-inflammation to get clear of the eyes. Your father should have the good rest for the eyes. He should also do the warm compress to make the eyes get comfortable and moisture.
It is really bad for people suffered from visual snow because they see things snowy like television-like static across their visual field. And so far, i didn't find any established treatment for visual snow. To treat vision snow, many patient teat it in migraine medication. It may help them a little. But i didn't find people who success from the treatment.